This invention relates to thin film magnetic recording media and more specifically to magnetic media covered with material which prevents corrosion, improves wear resistance, and reduces the head to media stiction coefficient.
Metallic magnetic thin film disks used in memory applications typically comprise a substrate material which is coated with a magnetic alloy film which serves as the recording medium. Typically, the recording medium used in such disks is a cobalt-based alloy such as Co-Ni, Co-Cr, Co-Ni-Cr, Co-Pt or Co-Ni-Pt which is deposited by vacuum sputtering as discussed by J. K. Howard in "Thin Films For Magnetic Recording Technology: A Review", published in Journal of Vacuum Science & Technology, in 1985, incorporated herein by reference, or a Co-P or Co-Ni-P film deposited by chemical plating as discussed by Tu Chen et al. in "Microstructure and Magnetic Properties of Electroless Co-P Thin Films Grown on an Aluminum Base Disk Substrate", published in the Journal Applied Physics in 1978, and Y. Suganuma et al. in "Production Process and High Density Recording Characteristics of Plated Disks", published in IEEE Transactions on Magnetics in 1982, also incorporated herein by reference. Several problems are encountered in using unprotected metallic thin film recording media. For example, unprotected metallic thin films tend to corrode, particularly under high humidity conditions. Further, such films have very little resistance to wear caused by frequent contact between the recording head and the media.
To prevent these problems, it is known in the art of recording technology that overcoating thin film magnetic recording media with a hard protective layer such as a carbon or SiO.sub.2 layer improves the wear resistance of the recording media and also provides some corrosion protection to the magnetic film in a low humidity and low temperature environment. Carbon overcoatings for magnetic disks are described by F. K. King in "Datapoint Thin Film Media", published in IEEE Transactions on Magnetics in 1981, and Japanese Patent Application No. 58140/77 filed May 18, 1977 by Hinata et al., incorporated herein by reference. It is possible, in principle, to increase the corrosion protection by increasing the thickness of the carbon or SiO.sub.2 overcoat. However, the maximum thickness of the overcoat that is tolerable for high performance disk media is about 2 microinches (2.mu."). Of importance, the performance of the disk is improved as the overcoat is made even thinner. The decrease in overcoat thickness decreases the "effective flying height" of the head on the media (i.e. the gap between the surface of the head and the magnetic layer), thereby improving the signal to noise ratio (S/N), resolution, and overwrite characteristics of the recording media. Unfortunately, if the carbon or SiO.sub. 2 overcoat thickness is less than 2.mu.", the overcoat does not provide sufficient corrosion protection for the magnetic media.
To improve the corrosion protection of the magnetic media provided by carbon or SiO.sub.2, it is known in the art to deposit a thin chromium layer between the magnetic thin film media and the overcoat layer. In this multi-layer overcoat structure, the chromium layer provides enhanced corrosion resistance while the carbon or SiO.sub.2 overcoat provides good wear resistance. However, in order to have effective corrosion resistance as well as good wear resistance provided by the carbon-chromium or SiO.sub.2 -chromium multi-layer structure, the total overcoat thickness must be greater than 2.mu.", which is not desirable for a high performance disk.
As mentioned earlier, the overcoat must not only protect the magnetic film from corrosion, but it must also protect the magnetic film from wear. A further requirement is that the stiction coefficient between the read/write head and the overcoat must remain low over a large number of stop/start cycles. (The stiction coefficient is a ratio of lateral force and the normal loading force on the head as the disk starts to rotate.) If the stiction coefficient between the head and the overcoat material is too high (greater than 1.0), a small motor used in the drive will have difficulty starting rotation of the disk from a stationary position, and if a large motor drives the disk, the motor may cause the head to break off from the head suspension. When the disk is rotating in the drive, the head "flies" at a typical distance of about 5 to 15.mu." above the disk. When the drive is turned off, the head comes into physical contact with the disk. Since the drive is likely to be repeatedly turned on and off during its lifetime, the overcoat must protect the magnetic film from wear, and at the same time, the stiction coefficient between the head and the overcoat must remain low after repeated start/stop cycles It has been demonstrated that even though hard carbon or SiO.sub.2 overcoats resist wear well, the stiction coefficient increases dramatically after repeated start/stop cycles.
Because of the above-described mechanical and corrosion problems, it would be desirable to coat a magnetic disk with an overcoat material which would improve corrosion protection of the magnetic film without being excessively thick and at the same time have good wear resistance and a low stiction coefficient.